The present invention relates to a device for monitoring the light output of a vertical cavity surface emitting laser (VCSEL) and a recording/reproducing optical pick-up adopting the device, and more particularly, to a device for monitoring the light output of a VCSEL, which is improved to efficiently monitor the intensity of light emitted from a VCSEL light source, and a recording/reproducing optical pick-up adopting the device.
In general, a device for controlling the light output of a VCSEL includes a monitoring optical detector used as means for controlling the intensity of light emitted from a VCSEL light source.
A VCSEL light source is different from an edge emitting laser light source, in that the former emits light vertically from a stack of semiconductor materials deposited on a substrate, while the latter emits light laterally therefrom. Properties of the VCSEL light source include circular light output, high light intensity, and single mode operation. These advantages make the VCSEL attractive for optical applications such as a light-output device for an optical pick-up, or a computer.
However, since the lower surface of the VCSEL is combined with a semiconductor substrate and thus light is vertically emitted from the upper surface thereof, it is difficult to install a monitoring optical detector.
To overcome this problem, a "Feedback mechanism for VCSEL" is disclosed in U.S. Pat. No. 5,285,466.
This mechanism will be described referring to FIGS. 1-3.
The mechanism is provided with a VCSEL light source 12 for emitting light by application of a forward biased voltage, and an annular monitoring optical detector 14 installed around light source 12, for absorbing spontaneous light emitted laterally from light source 12. Light source 12 and optical detector 14 are integrated on a single semiconductor substrate 10. Optical detector 14 usually employs a VCSEL, and receives light by having no voltage or a reverse biased voltage applied to electrode layers formed on the upper and lower surfaces thereof.
Optical detector 14 receives the lateral light emitted from light source 12, converts the light into an electrical signal, and feeds the signal back to an electrode of light source 12, thereby controlling light emission of light source 12.
However, the intensity of the lateral light emitted from light source 12 is not directly proportional to and is smaller than that of vertical light emitted therefrom. Further, much of the emitted lateral light penetrates through optical detector 14, and is not absorbed therein. Therefore, it is difficult to obtain the light intensity necessary to generate a signal strong enough for controlling the intensity of the vertical light.
As shown in FIG. 2, the vertical light is generated and emitted from light source 12 by forward biased lasing current in mA units. In an analysis of vertical light detected in an optical detector, it is found that there are a lasing threshold 20 and a lasing terminates 22 in the vertical light. When a lasing current between lasing threshold 20 and lasing terminates 22 is applied, a gaussian-shaped output current 24 of several microamperes is generated in the detector, while when any other current is applied, the detected current is almost zero in the detector.
The current amount 34 of lateral light emitted from light source 12 and detected in optical detector 14 is smaller than that of the vertical light emitted from light source 12. A lasing threshold 30 and a lasing terminates 32 of the lateral light are indefinite, and the detector current steadily increases and then decreases according to the strength of applied current. Therefore, since the intensity of the lateral light emitted from VCSEL light source 12 and detected in optical detector 14 is much smaller than that of light emitted from an edge emitting laser light source and detected in a monitoring optical detector, accuracy of light output control is likely to decrease due to noise.
FIG. 3 is a simplified view of another example of a conventional device for controlling the light output of a VCSEL. To overcome the problems described with reference to FIGS. 1 and 2, this device is comprised of at least two VCSELs 50 and 51 provided in parallel on a single semiconductor 40 of, for example, an N-type GaAs, and an identical forward biased voltage 52 is applied to VCSELs 50 and 51. Here, VCSELs 50 and 51 both emit light in a vertical direction and in a horizontal direction, at the same time, and the light of VCSEL 50 is proportional to the light of VCSEL 51, due to application of an identical forward biased voltage.
Here, one VCSEL 50 is used as a light source, and the other VCSEL 51 provided with a current sensor 56 where a reverse biased voltage 54 is applied, is used as an optical detector. Therefore, light emission of VCSEL 50 used as the light source is controlled by detecting light emitted from the surface of VCSEL 51. In this case, it is easy to control light emission. However employment of two VCSELs increases the bulk and cost of the apparatus.